This invention relates to a heat exchange method and apparatus in which a downflow reboiler is employed to boil liquefied gas.
In a conventional cryogenic air separation process the air is rectified in a double rectification column comprising a higher pressure column and a lower pressure column. The two columns are thermally linked by a reboiler-condenser. The reboiling passages of the reboiler-condenser are normally arranged so as to boil liquid oxygen in the sump of the lower pressure column. The necessary heating is provided by nitrogen separated in the higher pressure column. The nitrogen flows through the condensing passages of the reboiler-condenser and is thereby condensed.
The reboiler-condenser is typically at least partially immersed in liquid oxygen in the sump of the lower pressure column. A thermosiphon effect causes passage of the liquid oxygen through the reboiling passages of the reboiler-condenser. Because of the effect of the head of liquid oxygen in the sump there is not an uniform temperature difference from top to bottom of the reboiler-condenser between the boiling liquid and the condensing nitrogen vapor. Accordingly, the average temperature difference is significantly greater than the minimum temperature difference, and the average temperature at which the oxygen boils is similarly greater than it would be at the minimum temperature difference. In consequence, there is an increased consumption of power by the process.
In order to overcome this problem, increasing use is being made of downflow reboilers in which the liquid oxygen to be boiled is fed to the top of the reboiler-condenser and flows downwardly through the boiling passages under gravity. The effect of the head of liquid oxygen in the sump of the lower pressure column is thereby eliminated. One known form of downflow reboiler is disclosed in U.S. Pat. No. 4,599,097.
Typically, the top of the downflow reboiler is located above the volume of liquid oxygen which is held in the sump of the lower pressure column. There is therefore a need to transfer liquid oxygen to be boiled from the sump to the top of the downflow reboiler. A conventional mechanical pump driven by an electric motor can be used for this purpose. Such a pump adds to the cost and complexity of the air separation plant. It has been proposed in AU-A-59857/90 to avoid having to provide such a motor-driven pump by employing a vapor lift pump instead. In a vapor lift pump the effective density of the liquid being transferred is reduced by vaporizing a part of the liquid. If sufficient of the liquid is vaporized the head of liquid in the sump of the low pressure column is sufficient to transfer the liquid from the sump to the top of the downflow reboiler. In AU-A-59857/90 there is disclosed using an auxiliary heat exchanger in order to heat the liquid oxygen being transferred and thereby vaporize some of it. A disadvantage of this arrangement is that there may not be sufficient heat conveniently available to drive the circulation.
It is an aim of the present invention to provide a heat exchange apparatus and method which avoids the use of a motor driven pump to transfer liquid from a sump to a downflow reboiler while at the same time avoiding the need to provide a suitable heating medium.